This invention relates to switching regulators. More specifically, this invention relates to control circuits and methods for controlling buck-boost switching regulators for maintaining high efficiency.
A switching regulator provides a regulated output voltage V.sub.OUT to a load from an unregulated input voltage V.sub.IN. A synchronous switching regulator has at least two switches that switch ON and OFF out of phase with each other to supply current to a load. A control circuit controls the switching of the switches.
Referring to FIGS. 1A-1C, three prior art synchronous switching regulators are described. FIG. 1A illustrates a typical buck switching regulator 10, which may only regulate an output voltage V.sub.OUT that is lower than input voltage V.sub.IN. FIG. 1B shows a typical boost regulator 12, which may only regulate an output voltage V.sub.OUT that is higher than V.sub.IN. FIG. 1C illustrates a typical buck-boost switching regulator 14, which may regulate an output voltage V.sub.OUT that is higher, lower, or the same as input voltage V.sub.IN.
Referring to FIG. 1A, synchronous buck switching regulator 10 has two switches A and B. A control circuit (not shown) switches A and B ON (closed) and OFF (opened) out of phase with each other to supply current to load 19. Switching regulator 10 includes input capacitor 16, synchronous switches A and B, inductor 17, and output capacitor 18. Input voltage source V.sub.IN and input capacitor 16 are coupled between a first terminal of switch A and GROUND. Switch B is coupled between a second terminal of switch A and GROUND. A first terminal of inductor 17 is coupled to the second terminal of switch A, and output capacitor 18 and load 19 are coupled between a second terminal of inductor 17 and GROUND.
Referring to FIG. 1B, synchronous boost switching regulator 12 has two switches C and D. A control circuit (not shown) switches C and D ON (closed) and OFF (opened) out of phase with each other to supply current to load 19. Switching regulator 12 includes input capacitor 16, synchronous switches C and D, inductor 17, and output capacitor 18. Input voltage source V.sub.IN and input capacitor 16 are coupled between a first terminal of inductor 17 and GROUND. Switch C is coupled between a second terminal of inductor 17 and GROUND. Switch D has a first terminal coupled to the second terminal of inductor 17, and a second terminal coupled to a first terminal of output capacitor 18. Output capacitor 18 has a second terminal coupled to GROUND, and load 19 is coupled between the first terminal of output capacitor 18 and GROUND.
Referring to FIG. 1C, synchronous buck-boost switching regulator 14 includes input capacitor 16, inductor 17, output capacitor 18, and switches A, B, C, and D. Switches A, B, C, and D may, for example, be metal oxide semiconductor field effect transistors (MOSFETs) or bipolar junction transistors (BJTs). Input voltage V.sub.IN and input capacitor 16 are coupled between a first terminal of switch A and GROUND. Switch B is coupled between a second terminal of switch A and GROUND. Inductor 17 is coupled between the second terminal of switch A and a first terminal of switch D, and switch C is coupled between the first terminal of switch D and GROUND. Output capacitor 18 and load 19 are coupled between a second terminal of switch D and GROUND.
Switching regulator 14 includes four switches (A, B, C, and D). A control circuit (not shown) switches A, B, C, and D ON and OFF to supply current to 15 load 19. Prior art control circuits typically switch A and C ON together and B and D ON together. Switches A and C are OFF when switches B and D are ON, and switches B and D are OFF when switches A and C are ON.
Prior art control circuits use the following repeating switching sequence: A and C ON, then B and D ON, then A and C ON, then B and D ON, etc. Thus, prior art control circuits switch all four switches ON and OFF in regulator 14 to supply current to load 19.
An example of a prior art control circuit that may be used with the regulators of FIGS. 1A-1C includes a pulse-width modulator that has a single comparator that compares a control voltage at its non-inverting input with a symmetric triangular (or asymmetric sawtooth) waveform signal at its inverting input to generate a digital pulse-width modulated signal. The control voltage is generated from the output voltage of the regulator. As the control voltage is swept from the bottom to the top of the waveform signal, the duty cycle of the pulse-width modulated signal increases from 0% to 100%. In a buck-boost regulator, the pulse-width modulated signal is used to drive switches A and C together, while the inverse of the pulse-width modulated signal is used to drive switches B and D together. The control voltage varies the duty cycle of the pulse-width modulated signal, and thus it also varies the input-to-output voltage ratio of the regulator.
Synchronous buck-boost regulators such as regulator 14 advantageously may be operated to provide a regulated output voltage over a wide variety of output-to-input voltage requirements. Prior art synchronous buck-boost switching regulator control circuits, however, disadvantageously always drive all four switches ON and OFF in each cycle to regulate V.sub.OUT, regardless of the output current and the output-to-input voltage ratio. More power is consumed driving the switches ON and OFF, than when the switches remain either ON or OFF. More power is consumed by synchronous buck-boost regulator 14 than by synchronous buck regulator 10 or synchronous boost regulator 12, because only two switches have to be driven ON and OFF in regulators 10 and 12. Therefore, synchronous buck-boost switching regulator 14 is less efficient to use with a prior art control circuit than either synchronous buck regulator 10 or synchronous boost regulator 12.
A further disadvantage of switching regulator 14 used with a prior art control circuit is that the average inductor current is high. High average inductor current is undesirable because more power is consumed in the inductor to regulate the output voltage. The relationship between the average inductor current I.sub.IND and the average output current I.sub.OUT of switching regulator 14 used with a prior art control circuit may be expressed as: ##EQU1## where V.sub.OUT is the output voltage and V.sub.IN is the input voltage of switching regulator 14. For example, when V.sub.IN =V.sub.OUT, the average inductor current is twice the average output current in switching regulator 14, assuming no loss.
It would, however, be desirable to provide a high efficiency buck-boost switching regulator control circuit that can regulate an output voltage that is higher, lower, or the same as the input voltage. It would also be desirable to provide a buck-boost switching regulator control circuit that conserves power by driving fewer than all of the switches when the input voltage is higher or lower than the output voltage. It also would be desirable to provide a buck-boost switching regulator that has a low average inductor current.